The invention relates to a method for the transmission of direct current between two static converters. It is also related to a closed-loop control arrangement for the transmission of direct current between two alternating-current systems.
In certain cases, direct-current transmission is used for transmitting electrical power between two alternating-voltage systems. In this arrangement, a rectifier is connected to the first alternating-voltage system, which rectifier generates a direct current which, in turn, is supplied to an inverter. The inverter is connected to the second alternating-voltage system. In this manner, the two alternating-voltage systems are coupled to one another via a direct-current connection. The direction of energy flow can be reversible in this arrangement. Such an arrangement involving direct-current transmission between two static converters is known, for example, from WO 92/22 118.
To operate such arrangements, different closed-loop control methods are used. An example of these is the so-called marginal-current method. In this method, rectifier and inverter in each case exhibit a current controller, the characteristics of which are offset from one another by a marginal current. It is also usual in this arrangement that both static converters exhibit a voltage controller. In this manner, an exactly defined operating characteristic which exhibits a current and a voltage branch is allocated to each static converter. The intersection between the respective operating characteristics then produces the operating point of the overall arrangement in operation. Depending on the instantaneous operating situation, only one controller, namely that for current or voltage is always active, if necessary via a minimum- or maximum-demand selection, in such an arrangement. The other controllers are not involved. When disturbances occur, for example due to harmonic superimpositions on the direct current, the instantaneous operating point can approach the nominal operating point or depart from it in accordance with the harmonic oscillations.
EP-A-0,087,40 discloses a method and a closed-loop control arrangement for the transmission of direct current between two static converters, the controller devices of the static converters in each case being supplied with a control signal. The static converters exhibit a common operating point. In order to form the respectively active control signals, in this case a selection is made via a respective maximum selection between two available control signals. In this process, one or the other control signal respectively comes into play. This method also operates according to the marginal-current principle.